In modern industrial applications, fluid pumps have found use in many applications where large bodies of fluids need to be transported from one location to another. Fluid pump systems allow fluid to be stored at a location remote from a source so it can then be distributed quickly and easily. For example, pump systems are commonly used to provide drinking water from storage wells, move water from a storage well to a fire, and to provide water to power generating plants. Fluid pumps are also used to remove undesirable fluid from various locations. For example, fluid pumps are used to remove wastewater, drain floodwaters, and eliminate stormwater.
In such applications it is imperative that the fluid to be pumped is directed to the pump quickly and without interruption and turbulence. If flow to the pump slows or is turbulent, the pump output and efficiency will be negatively affected. If air or gaps in the flow are present, the pump may fail or lose capacity. In the alternative, if smooth flow is provided to the pump at a proper rate the pump will operate smoothly and efficiently. In addition to decreasing operating costs by increasing pump efficiency and effectiveness, smooth intake flow to the pump lengthens the pump life and lowers maintenance costs.
A problem common to all these pumping applications stems from the formation of vortices or flow disruptions in the fluid during pumping. Because many such applications involve high-throughput pumps, it can be difficult to efficiently transport the fluid flow from the storage tank to the pump while avoiding flow problems. In particular, various components or features in the intake system along the flow path tend to disrupt the flow of the fluid.
Such disruptions yield uneven and inefficient fluid flow, with the likely development of vortices and vapor cavities and the loss of adequate energy at the pump inlet. Severe turbulence can lead to “whitewater,” which is the introduction of large quantities of air into the fluid, reducing pump capacity and causing mechanical disruption of the pump operation. Even mild flow distortions can disrupt normal pump operation. For example, introduction of vapor cavities or bubbles into the pump can cause vibrations in the pump that reverberate throughout the pump system.
Vortex types caused by nonuniform flow are generally categorized into six types depending on severity of flow disruption. By way of example, type 1, the most mild, is a surface swirl whereby the surface of the fluid has been disrupted but the swirl is not coherent. “Coherent” means the surface vortex is connected to subsurface vortices. Type 4 is a vortex that pulls contaminant solids from the surface through the vortex column. Type 5 occurs when air is pulled into the vortex. The most severe vortex, type 6, occurs when a full air core extends to the pump inlet.
Over the years, several methods have been developed to prevent the formation of vortices in the fluid and provide acceptable fluid characteristics at the pump inlet. Early methods for creating efficient flow utilized large structures and complex baffling systems. These designs are commonly referred to as ‘shoe-box’ intakes. Other designs use specially fabricated tubes, termed Formed Suction Inlets, or FSIs, attached to the pump inlet bell to characterize and direct the fluid properly toward the pump inlet. All of these intake structures are characterized by the presumption that the fluid entering the pump must approach the pump inlet at the level of the inlet from a substantial distance away from the pump. Fluid is drawn through the inlet structure from the bottom of the approach conduit or channel where the currents from the upstream conduit or canal are likely to encourage the development of vortices and cause unacceptable turbulence at the pump inlet.
These early methods facilitate pumping by moving the fluid from the bottom of the tank, but they do not fully overcome the problem of vortex formation and swirling exacerbated by the demands of modern applications. The structures and FSIs required by these methods prove to be expensive, requiring considerable excavation, dewatering and extensive construction activity to fabricate and install. Inevitably, the performance of these intake designs are adversely affected by local influences such as other pumps in operation or changes in direction of the fluid flow in the structure approaching the intake. All of these designs are predicated on the assumption that the fluid approaching the intake is uniform and free from disturbances that would result in swirling or high energy currents approaching the intake. Even though the fluid flow is well distributed approaching these intakes, the localized influences noted previously may result in swirling and a vortex may still result. In particular, swirls can be created at or near the surface and at the inlet to the pump. The swirls in turn lead to flow defects that degrade the performance of the pump. All these problems are exaggerated when the flow rate is increased, localized influences effect the uniformity of fluid approach conditions, and the like. In practice, nearly all such intakes require after the fact modification to add special features to defeat swirl and vortex formation as well as correction of tendencies to develop floor, wall and ceiling separation phenomena that further encourages turbulent condition at the pump inlet.
Swirls in most intake designs develop in several key areas. Most notably, as the fluid flows to the intake it accelerates, but not uniformly. Instead, the fluid tends to follow floors and walls in the intake structure and it is at these locations where the greatest rate of acceleration of the fluid is encountered. Vortices often develop because this acceleration is not uniform. Also, corners and non-uniform curves in the intake structure creates pockets that encourage eddies and vortices in the fluid flow. Second, at the bottom of the tank, vortices often develop as the fluid is abruptly redirected from a downward flow to an upward direction into the pump intake. This abrupt acceleration change causes vortices near the pump inlet. It also slows the feed rate to the pump.
Many intake designs require substantial submergence, usually twice the pump bell diameter, to suppress the formation of surface swirling that could develop into Type 3 or higher vortices.
Other methods have expanded upon the idea of feeding the pump through a tube by configuring the tank to direct the fluid to a mouth of the tube. Examples of several such conventional tank designs include U.S. Pat. No. 2,072,944 to Durdin, U.S. Pat. No. 5,435,664 to Pettersson, and U.S. Pat. No. 4,033,875 to Besik. Such tank designs include a sloping portion at a bottom of the tank which concentrates flow at the inlet to the pump intake tube. The sloped tank walls also accelerate fluid as it approaches the inlet of the pump intake tube. The increased pressure at the tube inlet alleviates the burden on the pump because it does not have to draw the fluid with as much force. However, such tank designs present the same flow problems as previous intake designs. Vortices and eddies near the intake tube opening are common given the high pressure forcing fluid into the pump inlet.
Several modern methods have used knowledge of fluid transfer properties to redesign the pump intake assembly to try to create uniform flow. One method disclosed in U.S. Pat. No. 5,833,434 to Stahle involves the submersion of the pump and an intake tube into a casing configured to enhance the swirling of fluid rather than prevent it. Stahle discloses a casing with channels that concentrate the swirling flow into a swirling vertical path towards the pump. Although this method directs the flow of fluid into the intake pump, the increased swirling leads to vortices that allow the introduction of air in the bottom of the tank and similar problems. The swirling beneath the surface of the fluid also involves shearing between different flow paths, which actually increases the likelihood of vortex formation.
Another intake tube design includes a pit extending downward from a bottom of the tank. The pit is positioned near the center of the tube in order to prevent flow problems from interactions of the flow with the tank walls. However, the fluid still has a tendency to swirl as it moves down the pit to the intake tube inlet. The sharp corners formed by the upper lip of the pit at the bottom of the tank floor also create an additional location for the formation of vortices and the sharp corners do not have any effect on the potential for mal-distribution of fluid around the cross-section of the pit. In order to overcome this problem, one intake assembly design includes a weir surrounding the pit entry. The weir acts as a flow distributor as well as a barrier to eliminate floor currents in the tank.
Such designs further act to isolate the incoming flow from the inlet to the pump inlet and more uniformly distribute flow as it moves near the pump inlet, but they do not eliminate the problems discussed above. Vortices are still likely to develop as the fluid is redirected into the intake tube unless the flow is slow over the weir. Typically, the flow must be at or below 1 foot/sec. over the submerged weir. The flow of fluid over the weir also tends to create small swirls that can attach to other flow disruptions and act on the pump.
What is needed is an intake assembly that overcomes the above problems. In particular, what is needed is an intake assembly that creates a uniform, distributed flow from an inlet in the tank all the way to the pump inlet bell. What is needed is an intake assembly that suppresses vortex formation in the fluid. What is needed is an intake tube assembly with fewer areas in which a vortex is likely to develop.
Further, what is needed is an intake assembly that allows for efficient, high-throughput operation of a pump. What is needed is an intake assembly that increases pump capacity. What is needed is an intake assembly that minimizes cost of construction, including excavation, yet achieves all of the above objectives.